In UMTS WCDMA there is a mechanism defined for Random Access transmission in the uplink. The UE (User Equipment or mobile station) transmits a randomly-selected preamble signal characterised by the following parameters:                Signature sequence (i.e. bit sequence),        Scrambling code,        Sub-channel (i.e. timing of the access slot in the frame).        
If the base station receives the signature, it acknowledges it on the Acquisition Indicator Channel (AICH) with a corresponding signature. If the UE receives a positive acknowledgement it transmits a message part on a RACH data channel.
If the UE does not receive an acknowledgement after transmitting the preamble signal, it can retransmit with a higher power using a procedure known as power ramping, usually comprising increasing power in predetermined steps.
If the UE still does not receive an acknowledgement after a number of steps (or receives a negative acknowledgement indicated by an inverted signature on the AICH) it can make another attempt following a further random selection of the above parameters.
If two or more UEs select the same signature and scrambling code in the same access slot, a collision occurs, and one or more of the UEs will have to restart the access procedure. The probability of collision depends on the number of Signature sequences, Scrambling codes, Access slots from which the UEs make their selection for the randomly-selected preamble signal.
According to known procedures, if a UE receives a NACK or a collision occurs, it has to start the power ramping procedure again from the beginning with the next-selected preamble. Typically the initial transmit power is quite low, usually derived from an open-loop power estimate, and repeating the ramping procedure can result in significant delay for the UE to access the network.